A metal-oxide-semiconductor field-effect transistor (MOSFET) uses an insulated gate to control current flow between a source and a drain of the MOSFET. Current Voltage characteristics of a conventional MOSFET are shown in FIG. 1. In FIG. 1, the horizontal axis represents voltage from the drain to the source (Vds). The vertical axis represents current values flow from the drain to the source (Ids). As long as the MOSFET is forward biased (Vds is positive), the gate-to-source voltage (Vgs)—sometimes called gate voltage Vg—controls current flow (Ids) through the MOSFET. The threshold voltage (Vth) is the minimum value of Vgs that is needed to create a conducting path between the source and the drain. As illustrated in FIG. 1, increasing the gate voltage above the threshold voltage results in increased conductivity.
When the MOSFET is negative biased (Vds is negative), the gate-to-source voltage (Vg) has less impact on current flow through the MOSFET. This is the result of a body diode intrinsic within FETs which allows current flow from source to drain regardless of the gate voltage. For example, in an n-channel MOSFET, the source and the drain are n+ regions and the body is a p region. The p-n junction formed at the intersection of the p body and the n+ regions act as a diode between the body and the source of the MOSFET and between the body and the drain of the MOSFET. Because in a MOSFET the source is typically shorted to the body, the body diode between the body and the source is irrelevant. However, the body diode to the drain allows a current path from the body to the drain when the MOSFET is negative biased (Vds is negative).